Feature #2012

Set event_flags on thread creation if hook exists

=begin
In ruby-debug, when a user sets a breakpoint that ends up executed by a later-created thread, then nothing will happen.

The only solution that I have at this point is to always set RUBY_EVENT_VM for all living threads every time the hook proc is called. I've tried to optimize this and failed; the cost of figuring out accurately if there's a new thread is always greater than the brute-force method of just always forcing the flags to be updated.